Beta-Ketoacyl-acyl carrier protein synthetase is the enzyme which elongates fatty acyl residues. This enzyme has been purified from E. coli and crystallized. This proposal is concerned with the chemical characterization of the subunits of the enzyme. Information concerning the identity of the subunits will come from characterization of amino and carboxyl terminii and from preferential labelling of the active site. The radioactive labelled acyl enzyme intermediate will then be treated with pepsin and the resulting peptides isolated and purified. The sequence and numbers of these labelled peptides should yield conclusive evidence concerning the identity of the subunits. The role of the tryptophan residues will be examined by fluorescence measurements on the interaction of fatty acyl-ACP and free ACP with active and alkylated enzyme. Other peptides derived from ACP will be utilized as acyl carriers, should this system prove fruitful, in order to study the specificity of interaction of ACP and this enzyme. Further, chemical modifications of the enzyme will be employed in order to gain information on the amino acid residues which participate in the enzymatic reaction. Since the condensation reactions of fatty acid biosynthesis in all organisms thus far examined share certain common features, detailed structural knowledge of the E. coli enzyme should be useful in understanding fatty acid biosynthesis in other systems.